WO2002063758A1

WO2002063758A1 - A converter device and a method for the control thereof

Info

Publication number: WO2002063758A1
Application number: PCT/SE2002/000066
Authority: WO
Inventors: Bo Bijlenga; Falah Al-Hosini; Peter Lundberg; Gunnar Asplund
Original assignee: Abb Ab
Priority date: 2001-02-07
Filing date: 2002-01-16
Publication date: 2002-08-15
Also published as: SE0100372L; SE0100372D0; SE521243C2; EP1364450A1

Abstract

A converter apparatus for converting direct voltage into alternative voltage and conversely comprises a first VSC-converter (6) in cascade connection with at least one second VSC-converter (5). It has also a unit adapted to control the semiconductor devices of the converters and thereby the converter apparatus to generate a phase voltage being constituted by the sum of the voltages generated in said first and second VSC-converters. The first VSC-converter has between the positive and negative pole (7, 8) thereof a direct voltage being substantially higher than the direct voltage of the second VSC-converter (5) between its positive and negative pole.

Description

A converter device and a method for the control thereof

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a converter apparatus for converting direct voltage into alternating voltage and conversely, which comprises a first VSC-converter in cascade connection with at least one second VSC-converter, each VSC-converter of the apparatus comprising on one hand a direct voltage intermediate link, having a positive and a negative pole and one or more members for capacitive energy storage, and on the other current valves having controllable semiconductor devices, in which the apparatus comprises a unit adapted to control the semiconductor devices to generate voltages between the connection points of the respective VSC-converter mutually , separated in steps with a size of the direct voltage between the positive and the negative pole of the direct voltage intermediate link of the converter, and in which the unit is adapted to control said semiconductor devices and thereby the converter apparatus to generate a phase voltage constituted by the sum of said voltages generated in said first and said second VSC- converters, as well as a method for the control of such an apparatus. Such converter apparatuses may be used in all kinds of situations where direct voltage is to be converted into alternating voltage and conversely, in which examples of such uses are in stations of HVDC-plants (High Voltage Direct Current), in which direct voltage is normally converted into a three phase alternating voltage or conversely, or in so called back-to-back-stations where an alternating voltage is firstly converted into direct voltage and this is then converted into alternating voltage, as well as in SVC's (Static Var Compensator), where the direct voltage side consists of one or more capacitors hanging freely.

The invention is not restricted to any levels of the voltage of the alternating voltage side of the apparatus, the powers that the converter apparatus may transmit or the number of phases of the alternating voltage side of the apparatus, and it may ac- . cordingly very well be designed to generate a one-phase alternating voltage, for example for feeding of railway vehicles.

However, the invention is particularly, but not exclusively, di- rected to intermediate and high voltages, i.e. where the peak voltage of the alternating voltage side of the apparatus is 10 kV or higher.

By the cascade connection of at least two VSC-converters of an apparatus of this type the apparatus permits obtaining of comparatively many different levels on the alternating voltage side thereof, which in its turn means that comparatively fine curve shapes of the alternating voltage out from the apparatus may be obtained without any necessity to switch the controllable semiconductor devices included in the converters with - particularly high frequencies for that sake. Expressed otherwise, it gets possible to obtain a certain quality of the alternating voltage out from the apparatus by a lower switching frequency of the controllable semiconductor devices at a pulse width modulation pattern for controlling these than if for example a two level converter would be used. Thus, this means lower losses in the converter apparatus. Is instead the same switching frequency as for a two level converter used the curve shape of the alternating voltage may be made considerably better.

Already known converter apparatuses of this type have been used in one-phase configuration and also in three-phase configuration by connecting three said cascade connections either in Δ- or Y-connection. It is illustrated in Fig 1 what such an apparatus already known in three-phase configuration with three said cascade connections connected in Y-connection looks like. Each phase 1 , 2, 3 has n one-phase converters 4, 5, 6 in cascade connection having each two branches 9, 10 of two current valves 1 1 -14 connected in series connected in parallel between two direct voltage poles 7, 8 belonging to the respective con- verter, said current valves comprising a controllable (of turn-off type) semiconductor device 15 and a rectifying member 16 connected in anti-parallel therewith, such as a rectifying diode, with a midpoint of one branch 10 of a converter connected with the midpoint of a branch 17 of a subsequent converter 5 in the cas- - cade connection. The cascade connection of the one-phase converters is at one end at a phase reactor 19 connected to the alternating voltage phase 1 , while the cascade connection is at the other end connected to a point 20 in a Y-connection in common with the other phases. The direct voltage poles of the re- spective VSC-converter receive voltage through a direct voltage source illustrated in the form of a capacitor 21 -23. "The direct voltage pole" in the claims is defining that there is some type of direct voltage source connected to or included in the converter, which is expressed by "one or more members for capacitive en- ergy storage".

The different direct voltage sources 21 -23 deliver the same voltage to the respective VSC-converter, and it is in this way possible to obtain 2n+1 different levels of the voltage of the respec- ^" tive cascade connection on the alternating voltage side. Thus, in the case of three converters in cascade connection with each other 7 different levels of the voltage may be obtained, namely - 3U, -2U, -U, 0, +U, +2U, and +3U, if U is the voltage level between said direct voltage poles of the respective VSC-converter. So many levels means a "fine" curve shape of the alternating voltage without using a high switching frequency of the controllable semiconductor devices 15. One controllable semiconductor device and a diode are for the rest shown for each current valve in the figure, but these are intended to be symbols for a possibly larger amount of such members connected in series and adapted to function as one single, i.e. the controllable semiconductor devices connected in series in such a current valve are intended to be controlled simultaneously for functioning as one single such device.

An apparatus of the type defined in the introduction is already known through for example US patent 5 673 189.

Even if the construction of converter apparatuses of this type already known has the advantages mentioned above there are of course desires to improve them further, especially with respect to the possibilities to obtain an improved curve shape of the voltage on the alternating voltage side while utilizing a certain number of VSC-converters in cascade connection and to ^" bring down the switching losses and if possible also the cost for components included in the converter apparatus.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a converter ap- paratus of the type defined in the introduction, which is able to fulfil the desires just mentioned in a not negligible degree.

This is according to the invention obtained by the fact that in such a converter apparatus the first VSC-converter has between its positive and negative pole a direct voltage being substan- tially higher than the direct voltage of the second VSC-converter between the positive and negative pole thereof.

By the new approach to use different levels of the voltage be- tween the direct voltage poles of the VSC-converters totally new possibilities to optimize the function of such a converter apparatus are offered. The number of possible levels of the voltage out on said alternating voltage side may be increased, so that less VSC-converters are needed for obtaining an alternating voltage of a determined quality or at . a certain number of VSC- converters an improved curve shape of the alternating voltage may be obtained. Thus, costs may be saved in this way, either by using a lower number of components of the converter apparatus, or by less costly filters for filtering away noises of said alternating voltage. By the different level of the voltage between the direct voltage poles of the first and the second VSC-converter or -converters the possibility to improve the methods for controlling the converters and for example adapt the frequency by which the different converters are controlled to the level of said direct voltage is offered so as to reduce the switching losses and thereby increase the efficiency of the converter apparatus. The total efficiency of the converter apparatus may be raised and the harmonic content thereof may be improved. It is also possible to reduce the step between the voltage levels of said phase voltage and if desired minimize the number of switchings of the converter having the highest voltage between the poles thereof. These possibilities offered through a converter apparatus according to the appended patent claim 1 are the basis of an amount of preferred embodiments of the inven- tion defined in the appended dependent claims.

According to a preferred embodiment of the invention the current valves of the first VSC-converter have a plurality of semiconductor devices connected in series. By the fact that the first VSC-converter has a substantially higher direct voltage between the two poles thereof it is in many applications, especially when using the apparatus in stations in a transmission system, desired and also necessary that each current valve has a plurality of semiconductor devices connected in series, so that these are together able to hold the voltage to be held by the valve when it is blocked. However, it is well conceivable that the second VSC- converter or -converters have only one, or fewer, semiconductor devices connected in series in the current valves thereof than the first VSC-converter. Thus, the VSC-converters may be designed in different ways with respect to the number of semicon- ductor devices, and possibly also with respect to the properties of each individual semiconductor device, so that the existing substantially different level of the direct voltage between the poles of the converters may be utilized to an optimum.

According to another preferred embodiment of the invention the apparatus has a plurality of said second VSC-converters in cascade connection and all the VSC-converters of the cascade connection have mutually different voltages between the positive and negative pole thereof. The number of possible levels of the voltage out on the alternating voltage side of the converter apparatus for a certain number of VSC-converters of said cascade connection may in this way be increased remarkably with respect to such converters already known with the advantages mentioned above as a result.

According to another preferred embodiment of the invention the voltage U between the direct voltage poles of the VSC-converters may be expressed as U=kU₀, in which U₀ is a determined voltage level, k=a^"px, in which a is a positive number differing from 1 , in which p is >0 and x is the order number of the re- ^' spective VSC-converter -1 when the converters are associated with order numbers from 1 and upwardly. A stepwise change of the direct voltage level of the respective VSC-converter and possibilities to obtain many different levels of the total voltage out on the alternating voltage side of the converter apparatus by adding these in an appropriate way are hereby obtained. In a preferred embodiment of the invention p=1 and a=2, so that a binary-weighted converter apparatus is obtained with the direct voltages U₀, 1 /2U₀, 1 /4U₀, 1 /8U₀, 1/16U₀ and so on. This means that when there are m VSC-converters of the cascade connec- tion 2^{(m+1 )} -1 levels may be obtained, which is to be compared with 2m+ 1 levels of the prior art. This means in the case of m=3 that 15 different levels may be obtained instead of 7. These levels extend from -7/4U₀ to +7/4U₀ in steps of %U₀.

According to another very preferred embodiment of the invention the first VSC-converter is adapted to handle a substantially higher apparent power than said second VSC-converters. Through this construction of the apparatus the first VSC-converter may be controlled in a different way than the second VSC-converter for obtaining an amount of different objects, which among others appear from the further preferred embodiments of the invention discussed below. With respect to apparatuses of this type already known the switching frequency of the first VSC-converter with high apparent power may for exam- pie be reduced for obtaining lower switching losses and thereby a higher efficiency of the converter, while the second VSC-converter with a substantially lower apparent power may be switched with a higher frequency for obtaining a desired curve shape of the alternating voltage on the alternating voltage side of the apparatus.

According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, the relationship between the apparent power han- died by the respective second VSC-converter/the apparent power handled by the first VSC-converter is 0.10-1.0, in which said relationship is preferably 0.30-1.0 when the apparatus is designed for SVC-operation and 0.10-0.30 when the apparatus is designed for transmitting active power between the direct volt- age side and the alternating voltage side thereof. In the case of^' SVC-operation, i.e. when transferring reactive power, it may be advantageous that the relationship is high, since the higher this relationship the more contribution will be given by the second VSC-converter to the total apparent power of the apparatus, and the lower switching frequency may be used for the first VSC- converter with high apparent power. However, when active power passes the converter apparatus the second VSC-converter may usually not be used for increasing the total apparent power of the plant, but it is then mainly used for compensating away different disturbing harmonics generated by the first VSC- converter, in which it is then advantageous to let the apparent power of the second VCS-converter be considerably lower.

According to another preferred embodiment of the invention the apparatus has a first VSC-converter in the form of a three phase converter having three phase legs with controllable semiconductor devices between the two direct voltage poles thereof, one phase output of each phase leg is on the alternating voltage side thereof connected to a phase line, the apparatus has three said cascade connections with said first VSC-converter in com- mon for the cascade connections, a second VSC-converter of each cascade connection is at one end opposite to the alternating voltage side thereof connected to said phase output of a phase leg of the first VSC-converter each, and the second VSC- converters are formed by H-bridges with two branches of con- trollable semiconductor devices, a first one of which is connected to a phase leg of the first VSC-converter and a second one of which is connected to the alternating voltage side of the apparatus. This embodiment gives the advantage that three possible levels may be obtained for the voltage added by the second VSC-converter to the voltage from the first VSC-converter, so that in the case of one second VSC-converter per cascade connection 2x3=6 different levels may be obtained for the voltage pulses out on the alternating voltage side of the apparatus. According to an alternative embodiment, which coin- cides with the preceding one, except for the fact that each second VSC-converter is through the direct voltage side thereof connected to the phase leg of the first VSC-converter through a potential of the direct voltage intermediate link of the converter located substantially in the middle of the potential of the two direct voltage poles of this converter and connected to the alter- nating voltage side of the apparatus through a branch of controllable semiconductor devices, the total switching losses of the converter apparatus are certainly slightly increased, since the second VSC-converter may only provide two different levels, so that in the case of one second VSC-converter per cascade con- nection the number of different voltage levels will be four, but the advantage that the number of current valves of the converter apparatus to be controlled will be lower is instead obtained, so that costs for the components included therein may be saved.

According to another preferred embodiment of the invention the apparatus has a first VSC-converter in the form of a three phase converter having three phase legs with controllable semiconductor devices between the two direct voltage poles thereof, one phase output of each phase leg is on the alternating voltage side thereof connected to a phase line, in which this is obtained through the fact that each phase leg is connected to a secondary winding of its own of a transformer, the second end of the secondary winding is connected to a phase leg of a second VSC-converter in the form of a three phase converter, and the transformer has three primary windings, each one connected to a said phase line each of the alternating voltage side of the apparatus. An advantage of this embodiment is that only two direct voltage intermediate links are required, one for the first VSC- converter and one for the second VSC-converter, in spite of the fact that we are here talking about three phases, which simplifies the control of the converter apparatus. Furthermore, the fact that the direct voltage intermediate link capacitors for both converters are in common for the three phases means that the size of the direct voltage intermediate link capacitors may be chosen comparatively small, which reduces the costs for the converter apparatus. According to preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, the direct voltage side of the first VSC-converter is con- nected to a network for transmitting active power between the direct voltage side and the alternating voltage side of the apparatus, in which the direct voltage side of the first VSC-converter is preferably connected to a HVDC-transmission plant. It is in such a case particularly advantageous if a second three phase- VSC-converter is connected to the direct voltage side of the second VSC-converter with the midpoints of the phase legs thereof connected to a phase line each of an alternating voltage network for feeding power in towards and out from said second VSC-converter, respectively. The second VSC-converter, which has a substantially lower voltage between the direct voltage poles thereof than the first VSC-converter may hereby handle active as well as reactive power, since said further second three phase-VSC-converter with the alternating voltage network connection thereof means that the capacitors of the direct voltage intermediate link of the second VSC-converter may be kept charged on a desired level and not be discharged or charged too much for transmitting active power through the second VSC- converter. Accordingly, for a given level of the direct voltage on the direct voltage network the voltage on the alternating voltage side of the apparatus may be regulated upwardly or downwardly by controlling feeding of power in towards and out from, respectively, the second VSC-converter through said alternating voltage network if desired. If now the first VSC-converter has a considerably higher apparent power than the second VSC-converter and the converter with a high apparent power is connected to a transmission network for HVDC or functions as a back-to-back- converter, the converter having a low apparent power may then be used on one hand for reducing harmonics generated by the converter with a high apparent power and on the other for gen- erating a fundamental tone. The converter having a high apparent power may in this way use a pulse width modulation method with a very low switching frequency and with a fixed relationship between the alternating voltage and the direct voltage, while the converter having a low apparent power may be used for compensating harmonics, but also for reactive power compensation and/or for rapidly adjusting the total fundamental voltage of said converter apparatus on said alternating voltage side.

According to another preferred embodiment of the invention each phase leg of the first VSC-converter is connected to one phase line of the alternating voltage side of the apparatus of its own, and the apparatus has at least two second VSC-converters with each a connection to a direct voltage pole each of the first VSC-converter and a second connection to a pole conductor of a direct voltage network. This way to connect the VSC-converters to each other is particularly suitable in the case of HVDC, where the first VSC-converter on the direct voltage side is connected to an alternating voltage transmission network through reactors and filters without any intermediate transformer.

According to another preferred embodiment of the invention the apparatus has at least one dc/dc-converter having a high frequency transformer connected through one side thereof to said second VSC-converter and with the other side thereof to an arrangement for feeding power in towards and out from, respec- tiveiy, said second VSC-converter. This arrangement enables broadened possibilities of use of the second VSC-converter, both for contribution to reactive power compensation and transmission of active power in a similar way as for the embodiment discussed above with a further three phase-VSC-converter con- nected to an alternating voltage network.

According to another preferred embodiment of the invention the unit is adapted to control the semiconductor devices of said VSC-converter according to a pulse width modulation pattern with a frequency being the lower the higher the direct voltage between the direct voltage poles of the VSC-converter in ques- tion. This means in the practice that the VSC-converters of the apparatus having the lowest apparent power are switched with a higher frequency than converters with a higher apparent power, so that the total switching losses may be kept down and differ- ent types of semiconductor devices may be used in different converters for an optimum adaption to the intended switching frequency. It is pointed out that VSC-converters of the same apparatus are intended here and that in this those with a higher voltage between the direct voltage poles thereof are controlled with a lower frequency than those having a lower corresponding voltage. However, in such an apparatus with a converter with 5 kV between the poles thereof this may very well be controlled with a lower frequency than a converter with 20 kV between the poles of another apparatus.

According to another preferred embodiment of the invention said unit is adapted to control the first VSC-converter with a determined fundamental frequency and the second VSC-converters with a frequency being substantially higher, preferably a multiple of the fundamental frequency. This keeps the total switching losses of the converter apparatus on a very low level.

According to another preferred embodiment of the invention the unit is for obtaining said phase voltage adapted to keep the first VSC-converter in fixed switching positions during periods of time being as long as possible and during these periods of time control the semiconductor devices of the second VSC-converters to alternatively add different voltages to the voltage from the first - VSC-converter according to a pulse width modulation pattern. It is then particularly advantageous if the unit is adapted to control the VSC-converters according to a voltage set value for said phase voltage with the shape of a sine curve having a third tone component or a multiple of third tone components with respect to a fundamental tone of the sine curve added thereto for pro- longing said time the first VSC-converter may be present in a fixed position and does not have to be switched. Such an addi- tion of a third tone component, or an optional multiple of third tone components, does not influence the voltage between the phases, which accordingly will get a desired shape, which is known per se, but the number of switchings of the VSC-converter with high apparent power may be reduced further and the losses thereby be decreased. The increase of the efficiency means often in the practice that the apparent power of the converter apparatus may be raised thanks to a lower thermal load on the components included therein.

According to another preferred embodiment of the invention said unit is adapted to control the second VSC-converters to add the voltage to the voltage from the first VSC-converter for compensating away low frequency voltage harmonics generated as a consequence of the fact that the first VSC-converter is adapted to be located in a fixed position during great parts of the period of time of the fundamental tone voltage on the alternating voltage side of the apparatus.

According to yet another preferred embodiment of the invention the apparatus is designed for SVC-operation, i.e. for a reactive power compensation, and the unit is adapted to control the semiconductor devices of the other VSC-converters to generate voltage pulses having a fundamental tone being displaced with respect to the current through the converter by 90 electric degrees and to control the first VSC-converter with the same relationship between the voltage fundamental tone and the current through the converter for adding the contribution of the first and the second VSC-converters to a reactive power compensation. Advantages of utilizing the second VSC-converters in this way appear from the discussion above.

According to another preferred embodiment of the invention the apparatus is designed for transmitting active power between the direct voltage side and the alternating voltage side thereof, and said unit is adapted to control the semiconductor devices of the second VSC-converters for compensating away harmonics generated as a consequence of the operation of the first VSC-converter without giving any contribution to the transmission of active power.

According to another preferred embodiment of the invention said unit is adapted to only control semiconductor devices of two of the phase legs of the first VSC-converter at a time during parts of the period of the voltage fundamental tone of the converter and at the same time have the connection on the alternating voltage side of the third phase leg connected to one of the poles of the direct voltage intermediate link of the first VSC-converter and alternate between the three phase legs with respect to said connection to one of the poles at the transition between said pe- riod parts for applying a so called dead band-PWM on said VSC- converter, and the unit is adapted to at the same time control the VSC-converters according to a voltage set value for said phase voltage with the shape of a sine curve having a zero sequence component or zero sequence components, for example a third tone component or a multiple of third tone components, added thereto. The advantage of such a dead band-PWM is primarily that the switching frequency of the first VSC-converter, ^' which preferably is adapted to handle a high apparent power, then may be reduced to 2/3, since the phases only have to switch during 2/3 of the period of the fundamental tone. The disadvantage is that zero sequence components of third tone character or multiples of third tones have to be added to a voltage set value of all the phases, which does not influence the phase- phase-voltage but well the voltage between the phase and ground. However, thanks to the introduction of second VSC- converters with a low apparent power these zero sequence components may be compensated away, which leads to the possibility to introduce a dead band-PWM on the converter with a high apparent power without leading to any negative conse- quences for the voltage^' on the phase line with respect to ground. The invention also relates to a method for control of a converter apparatus as above, in which the semiconductor devices of said VSC-converter are controlled according to a pulse width modu- lation pattern having a frequency being the lower the higher the direct voltage between the direct voltage poles of the VSC-converter in question is. The advantages of this method as well as of embodiments of the method defined in the appended dependent claims appear without any doubt from the above discussion of preferred embodiments of the converter apparatus according to the invention.

The invention also relates to a computer program product as well as a computer readable medium according to the corre- sponding appended claims. It is easy to understand that the method according to the invention defined in the appended set of method claims is well suited to be carried out through program instructions from a processor influenceable by a computer ^' program provided the program steps in question.

Further advantages as well as advantageous features of the invention appear from the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description of preferred embodiments of the invention cited as examples.

In the drawings:

Fig 1 is a simplified circuit diagram of a converter apparatus according to a preferred embodiment of the invention,

Fig 2 is a view corresponding to Fig 1 of a converter apparatus according to a second preferred embodiment of the invention, Figs 3 and 4 illustrate a sinusoidal voltage set value and a voltage set value in the form of a sine curve having a third tone component added thereto for the voltage between the respective phase line and the first direct voltage intermediate link midpoint of the first VSC-converter in the converter apparatus according to Fig 2, which is utilized for pulse width modulation of the converter apparatus,

Fig 5 illustrates schematically what a pulse width modulation pattern starting from a voltage set value according to Fig 3 may look like for a converter apparatus according to Fig 2,

Fig 6 is a view corresponding to Fig 2 of a converter apparatus according to a third preferred embodiment of the invention,

Fig 7 a converter apparatus according to a fourth preferred embodiment of the invention, which constitutes a variation of the converter apparatus according to Fig 2,

Fig 8 is a view corresponding to Fig 2 of a converter apparatus according to a fifth preferred embodiment of the invention,

Fig 9 is a view corresponding to Fig 8 of a converter apparatus being a variation of the one shown in Fig 8,

Fig 10 is a view corresponding to Fig 8 of a converter apparatus according to a further variation of the converter apparatus according to Fig 8,

Fig 1 1 is a view corresponding to Fig 2 of a converter apparatus according to an eighth preferred embodiment of the invention, and Fig 12 is finally a view corresponding to Fig 2 of a converter apparatus according to a ninth preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A converter apparatus having a general construction being known per se and described above is illustrated in Fig 1 and has three cascade connections, one for each phase of the alternating voltage network, interconnected according to a Y-connection in the common point 20. However, the new fact of the present invention is that the voltage levels of the different direct voltage sources 21 , 22, 23 are different. More exactly, in a preferred embodiment, the level of the VSC-converter 4 is 2°U₀, in the one following thereupon 2^"1 U₀ and so on with 2^"XU₀ of the last VSC- converter 6 with x = the order number of the respective VSC- converter of the cascade connection -1 . We may now for the sake of simplicity assume that each cascade connection has only three one phase converters. The converter 4 may then between the connections 24 and 25 thereof deliver the voltage -U₀/4, 0 or +U₀/4 depending upon the state of the current valves 1 1 -14. The corresponding fact is valid for the one-phase- converter 5, which may deliver 0, 1 /2U₀ or -1 /2U₀ between the connection 24 and the connection 26 to the one-phase- converters 6 following thereupon. For that one-phase converter the levels 0,U₀ and -U₀ are in their turn valid. By adding these different combinations 15 different levels may be obtained: -7/4U₀ (-4/4 -2/4 -1 /4), -6/4 (-4/4 -2/4 +0), -5/4 (-4/4 -1/4 +0), -4/4 (-4/4 +0 +0), -3/4 (-4/4 +0 +1/4 or -1 /4 -2/4 +0) and so on - until +7/4U₀. The advantages of an apparatus of this type have been mentioned above.

Should 3 instead of 2 be used as base 3^m possible levels could be obtained for the respective cascade connection alternating voltage side, in which m is the number of one-phase-converters within the respective cascade connection. In the case of 3 one- phase-converters in each cascade connection for example 27 different levels may be obtained from -13/9U₀ to +13/9U₀ in steps of 1 /9U₀. As semiconductor devices of turn-off type in the one-phase-converter 6 with the highest voltage between the two direct voltage poles thereof is preferably such ones that may handle high powers used, but they are preferably operated at low frequencies, in which high frequency components are used as semiconductor devices of turn-off type in the one-phase-con- verter 4 with the lowest voltage between the direct voltage poles thereof and the frequency for the control of the semiconductor devices of the one-phase converters is increased in the direction from the one-phase-converter 6 to the one-phase-converter 4 for obtaining a desired pulse width modulation pattern (PWM) on the connection 25 to the reactor 19 of the alternating voltage side. IGBT's (Insulated Gate Bipolar Transistor) may then for example be used for higher switching frequencies and GTO's (Gate Turn-Off thyristor) for lower switching frequencies. The unit 27 for controlling the respective one-phase-converter, i.e. the power semiconductor devices 15 thereof, is designed to achieve this.

A converter apparatus according to a second preferred embodiment of the invention is shown in Fig 2, in which the first VSC- converter 6 here is present in the form of a three-phase-converter with three phase legs 28-30 with controllable semiconductor devices between the two direct voltage poles thereof (see furthest to the right in the Figure). A phase output of each phase leg is on the alternating voltage side thereof connected to a phase line 1 -3. This is achieved through a second VSC-converter 5, 5', 5" for each phase line, in which the second VSC- converter is formed by a H-bridge having two branches 31 , 32 of controllable semiconductor devices, one of which is connected to a phase leg of the first VSC-converter and the second of which to the alternating voltage side of the apparatus. We now assume that the voltage between the direct voltage poles 7, 8 of the first VSC-converter is U, then the voltage of the direct voltage intermediate link 33 of the second VSC-converters is k x U, in which k is substantially lower than 1 , preferably 0.05-0.5. This means that the apparent power of the second VSC-converters is low in relation to the apparent power of the first VSC-converter, since the same phase current I flows through the first and the second converters. The VSC-converters connected in series 5, 5', 5" are controlled according to a pulse width modulation pattern, in which they generate an alternating voltage between the input and the output thereof. The voltage between the input and the output may assume three discrete levels, namely k x U, 0 or -k x U. The voltage on the phase lines 1 , 2, 3 with respect to the midpoint 34, which defines the phase voltage, may assume totally 2 x 3 = 6 levels, compared with 2 levels for the case of us- ing only the first converter 6. The first VSC-converter has in this embodiment three connection points 35-37 on the alternating voltage side thereof, while the second VSC-converter has two connection points 38, 39 for each phase.

In reactive power compensation no active exchange with the surrounding alternating voltage network takes place more than for covering the losses on the network caused by the converter apparatus. This means that in reactive power compensation the second VSC-converter connected in series in each phase may be controlled for generating a fundamental voltage being 90 - electrical degrees phase shifted with respect to the fundamental tone of the phase current, exactly as for the first VSC-converter. The converter with low apparent power may in this way be controlled to give a contribution to the total reactive power of the converter apparatus. Furthermore, the second VSC-converter connected in series in each phase may be controlled to compensate away low frequency voltage harmonics generated as a consequence of the fact that the first VSC-converter does not switch during great parts of the period of the fundamental volt- age. The harmonics in question are primarily the fifth and seventh harmonics, the eleventh and thirteenth harmonics, but also higher harmonics or tones. In the case of reactive power the factor k may be chosen freely, preferably within the interval 0.15-0.5. If k for example is chosen to be 1 /3 the six voltage levels will be uniformly distributed, which may be particularly advantageous. The higher number chosen the greater contribution is given by the second VSC-converters connected in series to the total apparent power of the converter apparatus, and the lower switching frequency may be used for the first VSC-converter with a high apparent power.

The use of a sine curve 40 as a voltage set value for the phase voltage of the converter apparatus according to Fig 2 intended to form the basis for the pulse width modulation of the VSC-converters included therein is illustrated in Fig 3. The voltage levels U and -U , which may be obtained between the midpoint 34 and the connection point 35-37 of the respective phase leg on the alternating voltage side are shown, as well as the possible additions that may be made through controlling the second VSC- converters around the respective level, so that 41 corresponds to (1 /2+k)U, 42 to (1 /2-k)U , 43 to (-1 /2+k)U and 44 to (-1 /2-k)U. As long as the voltage set value is located between the levels 41 and 42 it is sufficient that the second VSC-converter having low apparent power connected in series in each phase switches. For the first VSC-converter with a high apparent power the phase output 35-37 thereof may during this period of time be connected to the positive pole 7. When the voltage set value is located between the levels 43 and 44 the phase output for the first VSC-converter is in corresponding way connected to the negative pole and the pulse width modulation switching is car- ried out for the second VSC-converter with low apparent power connected in series in the phase. Only during the rest of the time, see the arrow 45, it is necessary to switch the first VSC- converter with high apparent power. This means that the number of switchings to be carried out by the "big" converter with high apparent power during a period of the fundamental tone may be remarkably reduced, with lower switching losses as a conse- quence. The switching frequency of the second VSC-converter is typically in the region of 1-3 kHz.

An alternative possibility to the design of the voltage set value forming the basis for the pulse width modulation of a converter apparatus according to Fig 2 is shown in Fig 4. In this case a third tone component, which is here about 20 % of the fundamental tone, has been added to the voltage set value in all phases. Such an addition of a third tone component or an optional multiple of third tone components does not influence the voltage between the phases. Thus, the voltage set value of the phase-phase-voltage is still sinusoidal. This pulse width modulation method may advantageously be combined with the use of a second VSC-converter with low apparent power connected in series in each phase. It appears that the voltage set value of the phase voltage will now get steeper flanks, so that the period of time (the arrow 45) during which the voltage set value is located between the levels 42 and 43 is shortened in comparison with the corresponding period of time for the con- trol scheme according to Fig 3. The number of PWM-switchings to be carried out by the first VSC-converter with high apparent power during a period of the fundamental tone, which has usually a frequency of about 50 Hz, may by this be reduced further, which increases the efficiency of the converter. Furthermore, this modulation scheme results in a higher fundamental voltage out on the alternating voltage side for a given level of the direct voltage of the first VSC-converter, which also increases the efficiency of the converter apparatus and lowers the costs therefor.

It is illustrated in Fig 5 what the different voltage pulses on the alternating voltage side of the apparatus according to Fig 2 may look like in the case of reactive power when utilizing the control scheme discussed with reference to Fig 3. The converter apparatus according to Fig 2 is particularly well suited for reactive power compensation, but it may also be used for transmitting active power in the way described below.

If instead active power passes the first VSC-converter with high apparent power (for example in the case of HVDC or back-to- back-applications), the second VSC-converters connected in series in each phase may then not in the same way be used for increasing the total apparent power of the plant. These convert- ers may namely not contribute to the active power of the converter apparatus, since this would result in either a charging or a discharging of the direct voltage capacitor of the respective VSC-converter. However, the second VSC-converters connected in series may in this case be controlled for compensating away voltage components of for example the fifth and seventh harmonic, the eleventh and thirteenth harmonic and higher harmonics generated by the converter with low apparent power according to the above. In the case of active power the factor k is advantageously chosen to be low, for example 5-15%, since it is normally sufficient to add a small voltage component in series with the voltage from the big, first VSC-converter for generating and compensating away the harmonics mentioned above.

Thus, it is implied that in the case of active power the first VSC- converter as well as the smaller second VSC-converter connected in series in each phase are working with pulse width modulation. The higher number of available levels means that for a given requirement that the converter shall not generate more than a given amount of harmonics out on the connecting networks 1 -3 the switching frequency of the first VSC-converter 6 with high apparent power may be reduced.

A converter apparatus differing from the one according to Fig 2 only by the fact that the second VSC-converters are with the di- rect voltage side thereof connected to a phase leg of the first

VSC-converter through a potential of the direct voltage interme- diate link, which here has two capacitors, of the converter located substantially in the middle of the potential of the two direct voltage poles of this converter instead of being formed by H-bridges, and which is connected to the alternating voltage side of the apparatus to a branch of controllable semiconductor devices. By arranging such a half bridge only two voltage levels, namely kU/2 and -kU/2, respectively, may be added to the voltage of the first VSC-converter for obtaining the phase voltage. Thus, only four levels may be obtained for the phase voltage on the respective phase line 1 -3 with respect to the midpoint 34. The advantage of this embodiment is that the number of values to be controlled is reduced. The unit 27 may use the same control method as described above.

A further development of the embodiment according to Fig 2 is illustrated in Fig 7, which differs from the one in Fig 2 by the fact that each cascade connection has two second VSC-converters, so that it is here obtained that if for example k = 1/3 is chosen for the second VSC-converters 5, 5', 5" and k = 1 /6 is chosen for the second VSC-converters 4, 4' and 4",. 13 possible levels are totally obtained, which are evenly distributed between +U and -U in steps of U/6. Should instead k=1 /3 and 1 /9, respectively, be chosen as much as 18 levels may be obtained, which are evenly distributed to +17/18U and -17/18U in steps of 1/9U. The increased number of levels obtained in this way may be utilized for switching the current valves of the converters with a lower frequency for obtaining a given curve shape and in this way reduce the switching losses or switching the valves with an unchanged frequency and obtain an improved curve shape with less harmonic content.

An apparatus according to a further variation of the invention is shown in Fig 8, which is very suitable when the converter apparatus is connected to a connecting network 1 -3 through a trans- former 47. The first VSC-converter is here on the alternating voltage side thereof with each phase leg connected to a secon- dary winding 48-50 of its own of the transformer, and the second end of the secondary winding is connected to a phase leg of a second VSC-converter in the form of a three phase converter. The transformer has further three primary windings 51 -53, which are each connected to a said phase line 1 -3 each of the alternating voltage side of the apparatus. Thus, the transformer Y-connected on the secondary side is phasewisely provided with an extra lead-through in the neutral point 54 of the transformer, through which the second VSC-converter with low apparent power has been connected. In this type of connection four different levels per phase are obtained, but it is pointed out that this embodiment may be varied freely with the other embodiments according to the invention if more levels are desired. An advantage of this embodiment is that it only includes two direct volt- age intermediate links, which simplifies the control of the converter apparatus, and that the direct voltage intermediate link capacitors for both VSC-converters are in common for all the three phases, which makes it possible to select the size of the direct voltage intermediate link capacitors comparatively small, . which reduces the costs for the converter. The phase voltage is here present across the secondary winding of the transformer.

A variation of the embodiment according to Fig 8 is illustrated in Fig 9, which differs from the one according to Fig 8 by the fact that the first VSC-converter 6 with high apparent power on the direct voltage side thereof is connected to a transmission system for HVDC or alternatively directly to an identical station for a back-to-back-transmission, which is indicated through the cables 55, 56. Since the voltage between the direct voltage poles of the first VSC-converter now is assumed to be high also reactors 57 and filters 58 have been placed between this converter with high output voltage and the transformer 47 so as to avoid that the transformer is exerted to high voltage derivatives with respect to ground. A further modification of the embodiment according to Fig 8 is shown in Fig 10 and this differs from the embodiment according to Fig 9 by the fact that on the direct voltage side of the second VSC-converter 5 a further three phase-VSC-converter 76 is con- nected with the midpoints and the phase legs thereof connected to a phase line each of an alternating voltage network 60 for feeding power in towards and out from, respectively, said second VSC-converter 5 with lower apparent power. The converter 5 with low apparent power may in this way be used on one hand for reducing harmonics generated by the converter 6 with high apparent power and on the other for generating fundamental tone. The converter 6 with high apparent power may in this way use a pulse width modulation pattern with very low switching frequency and with a fixed relationship between alternating volt- age and direct voltage, while the converter 5 with low apparent power is used both for harmonic compensation and for reactive power compensation and/or rapid adjustment of the total fundamental voltage of the converter apparatus on the alternating voltage side.

A converter apparatus according to a further preferred embodiment of the invention is illustrated in Fig 1 1 , in this apparatus each phase leg of the first VSC-converter 6 is connected to a phase line 1 -3 of its own on the alternating voltage side of the apparatus and two second VSC-converters 5, 5' are connected to on one hand a direct voltage pole of the first VSC-converter each and on the other to a pole conductor of a direct voltage network. This embodiment of the invention is particularly suited in the case of HVDC, where the first VSC-converter on the alternating voltage side is connected to an alternating voltage transmission network 1 -3 through reactors 58 and filters 59 without any intermediate transformer. The second VSC- converters with low apparent power are preferably controlled synchronously with a pulse width modulation pattern, so that both either add or subtract the voltage kU with respect to the respective pole voltage in relation to ground. They may also be connected so that the pole voltage of the converter 6 with high apparent power gets identical to the voltage across the respective direct voltage capacitor with respect to ground. The current flowing through both VSC-converters 5, 5' with low apparent power is mainly a direct current. The voltage generated thereby is a pure alternating voltage without any direct voltage component. Since they are switching synchronously they will generate a zero sequence voltage being present in all phases on the alternating voltage side. The first VSC-converter 6 has here three connection points on the alternating voltage side thereof and two 72, 73 on the direct voltage side thereof, while the respective second VSC-converter 5 has two connection points 74, 75. The phase voltage for one phase is between 34 and 1 .

The converter apparatus according to this embodiment is well suited for use of so called dead band-PWM. During a given part of the period of the voltage fundamental tone only two of the three phases of the first VSC-converter 6 with high apparent power are in this way switched with their PWM pattern, while the third phase is connected to one of the direct voltage poles, 7, 8. It is for example possible to let one phase be connected to one direct voltage pole during 60 electrical degrees of the period of the fundamental voltage, whereupon the pole is switched during 120 electrical degrees, and the pole is then during 60 electrical degrees connected to the opposite pole, whereupon the pole is again switched during the remaining 120 electrical degrees. The advantage of dead band-PWM is as mentioned above primarily that the switching frequency of the VSC-converter with high ap- parent power may be reduced to 2/3, since the phase only have to switch during 2/3 of the period of the fundamental voltage. The disadvantage is on the other that zero sequence components having third tone character or multiples of third tones have to be added to the voltage set values of all the phases, which however does not influence the phase-phase-voltage but the voltage between phase and ground. However, thanks to the introduction of the two VSC-converters 5, 5' with low apparent power controlled synchronously these zero sequence components may be compensated away, which results in an introduction of dead band-PWM for the VSC-converter 6 with high apparent power without any negative consequences for the voltage on the respective phase line 1 -3 with respect to ground.

A typical value of the factor k may in this case be about 15-20%. Also higher values of the factor k may be used. This may for ex- ample be valuable if the VSC-converter with high apparent power is directly connected, i.e. without any transformer, to an alternating voltage transmission network being impedance grounded. For example on a one-phase ground fault a zero sequence component then appears on the alternating voltage side, inter alia of fundamental tone character. The VSC-converters with low apparent power may in such a case, provided that the factor k is selected sufficiently large, compensate this zero sequence component away and the converter apparatus may transmit power independently of any occurrence of one-phase faults in connecting networks.

Finally, an apparatus according to a further preferred embodiment of the invention is illustrated in Fig 12, and in this the second VSC-converters may also contribute to the transmission of active power by the fact that their direct voltage side may exchange energy with a further alternating voltage network 61 through a dc/dc-converter 62. This embodiment has a dc/dc- converter 62 with a high frequency transformer 63 connected with one side thereof to a second VSC-converter 5 with its other side to an arrangement (61 ) for feeding power in towards and out from, respectively, said VSC-converter. More exactly, the apparatus has a common dc/dc-converter for all the phase lines 1 -3 with a said transformer with three secondary windings 64-66 connected to a converter part 67-69 of its own connected to the respective second VSC-converter and a primary winding 70 connected to one single converter part 71 connected to said ar rangement. The additional network 61 may hereby feed power into or drain power from the second VSC-converters 5 with low apparent power, so that these may function in a similar way as the second VSC-converter 5 of the embodiment according to Fig 10.

The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing fro the basic idea of the invention as defined in the appended claims.

The embodiment lastly described may be modified by arranging a separate transformer/phase. However, it is advantageous to use a multiple winding transformer according to Fig 12, since the number of primary windings may then be reduced.

"Add voltage" is in this disclosure to be interpreted as also covering addition of negative voltages, i.e. a subtraction of a posi- tive voltage.

The converter apparatuses described are preferably designed to handle phase voltages between 5 kV and 500 kV, even if other voltage levels are conceivable.

Claims

1 . A converter apparatus for converting direct voltage into alternating voltage and conversely, which comprises a first VSC- converter (6) in cascade connection with at least one second VSC-converter (4, 5), each said VSC-converter of the apparatus comprising on one hand a direct voltage intermediate link (21 - 23, 33) having a positive and a negative pole and one or more members for capacitive energy storage, and on the other current valves (1 1 -14) having controllable semiconductor devices, in which the apparatus comprises a unit (27) adapted to control the semiconductor devices to generate voltages between the connection points of the respective VSC-converter mutually separated in steps with a size of the direct voltage between the positive and the negative pole of the direct voltage intermediate link of the converter, and in which the unit is adapted to control said semiconductor devices and thereby the converter apparatus to generate a phase voltage constituted by the sum of said voltages generated in said first and said second VSC- converters, characterized in that the first VSC-converter has between its positive and negative pole a direct voltage being substantially higher than the direct voltage of the second VSC- converter between its positive and negative pole.

2. An apparatus according to claim 1 , characterized in that said unit is adapted to control the semiconductor devices to utilize the relationship between the direct voltages of the first and second VSC-converters for in a favourable way generating a said phase voltage having a plurality of available voltage levels.

3. An apparatus according to claim 1 , characterized in that said unit is adapted to control the semiconductor devices to utilize the relationship between the direct voltages of the first and second VSC-converters for increasing the total efficiency of the converter apparatus.

4. An apparatus according to claim 1 , characterized in that said unit is adapted to control the semiconductor devices to utilize the relationship between the direct voltages of the first and second VSC-converters for improving the content of harmonics of the converter apparatus.

5. An apparatus according to claim 1 , characterized in that said unit is adapted to control the semiconductor devices to utilize the relationship between the direct voltages of the first and sec- ond VSC-converters to reduce the step between different possible voltage levels of said phase voltage.

6. An apparatus according to claim 1 , characterized in that said unit is adapted to control the semiconductor devices to utilize the relationship between the direct voltages of the first and second VSC-converters to minimize the number of switchings of the VSC-converter (6) having the highest voltage between the poles thereof.

7. An apparatus according to any of claims 1 -6, characterized in that the current valves of the first VSC-converter have a plurality of semiconductor devices (15) connected in series.

8. An apparatus according to any of claims 1 -7, characterized in that it has a plurality of said second VSC-converters (4, 5) in cascade connection with each other.

9. An apparatus according to claim 8, characterized in that all the VSC-converters (4, 6) of the cascade connection have mu- tually different voltages between the positive and negative pole thereof.

10. An apparatus according to claim 9, characterized in that the voltage U between the direct voltage poles of the VSC-con- verters may be expressed as U=kU₀, in which U₀ is a determined voltage level, k=a^"px, in which a is a positive number differing from 1 , in which p is >0 and x is the order number of the respective VSC-converter -1 when the converters are associated with order numbers from 1 and upwardly.

1 1 . An apparatus according to claim 10, characterized in that p=1.

12. An apparatus according to claim 10 or 1 1 , characterized in that a is an integer > 2.

13. An apparatus according to claim 12, characterized in that a = 2.

14. An apparatus according to claim 12, characterized in that a = 3.

15. An apparatus according to any of the preceding claims, characterized in that the first VSC-converter (6) is adapted to handle a substantially higher apparent power than said second VSC-converter (4, 6).

16. An apparatus according to claim 15, characterized in that the relationship between the apparent power handled by the respective second VSC-converter (5)/the apparent power handled by the first VSC-converter is 0, 10-1.

17. An apparatus according to claim 16, characterized in that it is designed for SVC-operation, and that said relationship is 0,30-1 .

18. An apparatus according to claim 16, characterized in that it is designed for transmitting active power between the direct voltage and alternating voltage side thereof, and that said relationship is 0, 10-0-30.

19. An apparatus according to any of the preceding claims, characterized in that it comprises a plurality of said cascade connections, which are at one end opposite to the alternating voltage side of the respective cascade connection connected to each other in a common point (20).

20. An apparatus according to claim 19, characterized in that three said cascade connections are connected to each other in said common point (20) for forming a Y-connection of three phases of the direct voltage side of the converter apparatus.

21 . A converter apparatus according to any of claims 1 -18, characterized in that it has a first VSC-converter (6) in the form of a three phase converter having three phase legs (28-30) with controllable semiconductor devices between the two direct voltage poles thereof, and that a phase output of each phase leg is on the alternating voltage side thereof connected to a phase line (1 -3).

22. An apparatus according to claim 21 , characterized in that it has three said cascade connections with said first VSC-converter (6) in common to the cascade connections, and that one second VSC-converter (5, 5', 5") of each cascade connection is at one end opposite to the alternating voltage side thereof con- nected to said phase output of a phase leg (28-30) each of the first VSC-converter.

23. An apparatus according to claim 22, characterized in that said second VSC-converters are formed by H-bridges having two branches (31 , 32) of controllable semiconductor devices, of which a first one is connected to a phase leg of the first VSC- converter and a second one is connected to the alternating voltage side of the apparatus.

24. An apparatus according to claim 22, characterized in that each second VSC-converter is either through the direct voltage side thereof connected to a phase leg of the first VSC-converter through a potential of the direct voltage intermediate link of the converter located substantially in the middle of the potential of the two direct voltage poles of this converter and connected to the alternating voltage side of the apparatus through a branch of controllable semiconductor devices or through said branch connected to said phase leg of the first VSC-converter and through said intermediate potential thereof to the alternating voltage side of the apparatus.

25. An apparatus according to claim 21 , characterized in that the first VSC-converter (6) is on the alternating voltage side thereof through each phase leg connected to an own secondary winding (48-50) of a transformer (47), that the other end of the secondary winding is connected to a phase leg of a second VSC-converter (5) in the form of a three phase converter, and that the transformer has three primary windings (51 -53), each connected to a said phase line (1 -3) each of the alternating voltage side of the apparatus.

26. An apparatus according to claim 25, characterized in that the direct voltage side of the first VSC-converter is connected to at least one capacitor hanging freely for SVC (Static Var Com- pensator)-operation of the apparatus.

27. An apparatus according to claim 25, characterized in that the direct voltage side of the first VSC-converter is connected to a network (55, 56) for transmitting active power between the direct voltage side and the alternating voltage side of the appa- ratus.

28. An apparatus according to claim 27, characterized in that the direct voltage side of the first VSC-converter is connected to a HVDC (High Voltage Direct Current)-transmission plant.

29. An apparatus according to claim 27, characterized in that the direct voltage side of the first VSC-converter (6) is through a direct voltage intermediate link connected to a VSC-converter with an alternating voltage side oppositely located connected to an alternating voltage network for back-to-back-transmission between the alternating voltage network and the alternating voltage side of the apparatus.

30. An apparatus according to claim 27, characterized in that a further three phase-VSC-converter (4) is connected to the direct voltage side of the second VSC-converter (5) with the midpoints of its phase legs connected to a phase line each of an alternating voltage network (60) for feeding power in towards and out from, respectively, said second VSC-converter.

31 . An apparatus according to any of claims 27-30, characterized in that a filter (59) is arranged on the alternating voltage side of the first VSC-converter (6) for filtering away harmonics ^" generated when switching the semiconductor devices of the converters.

32. An apparatus according to claim 21 , characterized in that each phase leg (28-30) of the first VSC-converter (6) is connected to a phase line (1 -3) of its own of the alternating voltage side of the apparatus, and that the apparatus has at least two second VSC-converters (5, 5') having each a connection to a direct voltage pole each of the first VSC-converter and a connection to a pole conductor of a direct voltage network (55, 56).

33. An apparatus according to claim 32, characterized in that said second VSC-converters (5, 5') are formed by H-bridges having two branches of controllable semiconductor devices, a first of which is connected to a direct voltage pole of the first VSC-converter (6) and a second is connected to a said pole conductor.

34. An apparatus according to claim 22, characterized in that it has at least one dc/dc-converter (62) having a high frequency transformer (63) connected through one side thereof to said second VSC-converter (5) and through its other side to an ar- rangement (61 ) for feeding power in towards and out from, respectively, said second VSC-converter.

35. An apparatus according to claim 34, characterized in that it has a separate said dc/dc-converter for each phase line for power exchange with the second VSC-converter belonging thereto.

36. An apparatus according to claim 34, characterized in that it has a dc/dc-converter (62) in common to all the phase lines (1 - 3) with a said transformer having three part secondary windings (64-66) connected to a converter part (67-69) each connected to a respective second VSC-converter and a primary winding (70) connected to one single converter part (71 ) connected to said arrangement.

37. An apparatus according to any of the preceding claims, characterized in that the unit (27) is adapted to control the semiconductor devices of said VSC-converter according to a pulse width modulation pattern with a frequency being the lower the higher the direct voltage between the direct voltage poles of the VSC-converter in question.

38. An apparatus according to claim 37, characterized in that said unit (27) is adapted to control the first VSC-converter (6) with a determined fundamental frequency and the second VSC- converters (4, 5) with a frequency being substantially higher, preferably a multiple of the fundamental frequency.

39. An apparatus according to any of claims 21 -38, character- ized in that said unit (27) is, for obtaining said phase voltage, adapted to keep the first VSC-converter (6) in fixed switching positions during as long periods of time as possible and during these periods of time control the semiconductor devices of the second VSC-converters (4, 5) to alternatively add different voltages to the voltage from the first VSC-converter according to a pulse width modulation pattern.

40. An apparatus according to claim 39, characterized in that said unit (27) is adapted to control the VSC-converters according to a voltage set value (40) for said phase voltage with the shape of a sine curve having a third tone component or a multiple of third tone components with respect to the fundamental tone of the sine curve added thereto for prolonging said period of time the first VSC-converter may be in a fixed position and does not have to be switched.

41 . An apparatus according to any of claims 21 -40, characterized in that said unit is adapted to control the second VSC- converters (4, 5) to add voltages to the voltage from the first VSC-converter (6) for compensating away low frequency voltage harmonics generated as a consequence of the fact that the first VSC-converter is adapted to be in a fixed position during large parts of the period of the fundamental voltage on the alternating voltage side of the apparatus.

42. An apparatus according to any of claims 21 -27 or 34-41 , characterized in that it is designed for SVC-operation, i.e. for compensating reactive power, and that said unit (27) is adapted to control the semiconductor devices of the second VSC-converters (4, 5) to generate voltage pulses having a fundamental tone being displaced with respect to the current through the converters by 90 electric degrees and to control the first VSC- converter (6) with the same relationship between the voltage fundamental tone and the current through that converter for adding the contribution of the first and the second VSC-con- verters to reactive power compensation.

43. An apparatus according to any of claims 21 -25 or 27-41 , characterized in that it is adapted for transmitting active power between the direct voltage side and the alternating voltage side thereof, and that said unit (27) is adapted to control the semi- conductor devices of the second VSC-converters (4, 5) for compensating away harmonics generated as a consequence of the operation of the first VSC-converter (6) without giving any contribution to the transmission of active power.

44. An apparatus according to claim 32 or 33, characterized in - that said unit (27) is adapted to control said second VSC-converters (5, 5') to switch synchronously.

45. An apparatus according to any of claims 21 -31 or 34-44, characterized in that said unit (27) is adapted to only control the semiconductor devices of two phase legs (28-30) of the first VSC-converter at a time during parts of the period for the voltage fundamental tone for that converter and at the same time have the alternating voltage side connection of the third phase leg connected to one of the poles (7, 8) of the direct voltage intermediate link of the first VSC-converter and alternating between the three phase legs with respect to said connection to one of the poles at transitions between said period parts for applying a so called dead band-PWM on said VSC- converter, and that said unit (27) is adapted to simultaneously control the VSC-converters according to a voltage set value for said phase voltage with the shape of a sine curve having a zero sequence component or zero sequence components, for example a third tone component or a multiple of third tone components, added thereto.

46. A method for controlling a converter apparatus according to claim 1 , characterized in that the semiconductor devices of said VSC-converter are controlled according to a pulse width modu- lation pattern having a frequency being the lower the higher the direct voltage between the direct voltage poles of the VSC-converters (4-6) in question.

47. A method according to claim 46, characterized in that the first VSC-converter (6) is controlled with a determined fundamental frequency and the second VSC-converters (4, 5) are controlled with a frequency being substantially higher, preferably a multiple of the fundamental frequency.

48. A method according to claim 46 or 47 for controlling a converter apparatus having for the rest a first VSC-converter (6) in the form of a three phase converter having three phase legs (28- 30) with controllable semiconductor devices between the two direct voltage poles thereof and with a phase output of each phase leg on the alternating voltage side thereof connected to a phase line (1 -3), characterized in that for obtaining a said phase voltage the first VSC-converter is kept in fixed switching positions during as long periods of time as possible and during these periods of time the semiconductor devices of the second VSC-converters (4, 5) are controlled to alternatively add different voltages to the voltage from the first VSC-converter according to a pulse width modulation pattern.

49. A method according to claim 48, characterized in that the VSC-converters are controlled according to a voltage set value

(40) for said phase voltage with the shape of a sine curve having a third tone component or a multiple of third tone components with respect to the fundamental tone of the sine curve added thereto for prolonging said period of time during which the first VSC-converter may be in a fixed position and does not have to be switched.

50. A method according claim 48 or 49, characterized in that the second VSC-converters (4, 5) are controlled to add voltages to the voltage from the first VSC-converter for compensating away low frequency voltage harmonics generated as a conse- quence of the fact that the first VSC-converter (6) is adapted to be in a fixed position during large parts of the period of the fundamental voltage on the alternating voltage side of the apparatus.

51 . A method according to any of claims 46-50, in which the apparatus is designed for SVC-operation, i.e. for reactive power compensation, characterized in that the semiconductor devices of the second VSC-converters (4, 5) are controlled to generate voltage pulses having a fundamental tone being displaced with respect to the current through the converter by 90 electric degrees, and that the first VSC-converter (6) is controlled with the same relationship between the voltage fundamental tone and the current through that converter for adding the contribution of the first and the second VSC-converters to reactive power compensation.

52. A method according to claims 46-50, in which the apparatus is adapted for transmitting active power between a direct voltage side and the alternating voltage side thereof, characterized in that the semiconductor devices of the second VSC-converters (4, 5) are controlled for compensating away harmonics generated as a consequence of the operation of the first VSC- converter (6) without giving any contribution to transmission of active power.

53. A method according to claim 46, which is intended to be applied to a converter apparatus having a first VSC-converter (6) in the form of a three phase converter having three phase legs with controllable semiconductor devices between the two direct voltage poles thereof and in which a phase output of each phase leg is on the alternating voltage side thereof connected to a phase line (1-3), characterized in that during parts of the period for the voltage fundamental tone of the first VSC-converter only - semiconductor devices of two of the phase legs (28-30) of this converter is controlled to switch at a time and at the same time the alternating voltage side connection of the third phase leg is kept connected to one of the poles (7, 8) of the direct voltage intermediate link of the first VSC-converter, in which it is alternated between the three phase legs with respect to said con- nection to one of the poles at transitions between said period parts, for applying a so called dead band-PWM on this VSC- converter, and that the VSC-converters are simultaneously controlled according to a voltage set value (40) for said phase voltage with the shape of a sine curve having a zero sequence component or zero sequence components added thereto, i.e. a third tone component or a multiple of third tone components.

54. A computer program product adapted to be loaded directly into the internal memory of a computer and comprising software code portions for instructing a processor to carry out the steps according to any of claims 46-53, when the product is run on a computer.

55. A computer program product according to claim 54 provided at least partially through a network as the Internet.

56. A computer readable medium having a program recorded thereon adapted to make a computer control the steps according to any of claims 46-53.